This invention relates to Vertical Cavity Surface Emitting Laser (VCSEL) arrays which are particularly suitable for optical tape recording.
A Vertical Cavity Surface Emitting Laser (VCSEL) can be configured to produced multiple, independent laser beams. The laser beams emerge as divergent beams from well defined apertures on the VCSEL device surface.
The VCSEL may be used as a source for multichannel recording, printing, optical data processing, or image display by imaging the VCSEL surface onto a medium of interest. However, the apertures of independent VCSEL sources are usually not formed closer together than about 75 xcexcm. Otherwise, electronic and optical cross-talk occurs. Since the VCSEL apertures are much smaller than the minimum spacing, when the source array is imaged onto a medium, the spots are also widely spaced.
FIG. 1 shows a prior art arrangement of a Vertical Cavity Surface Emitting Laser Array 10 having four independent VCSEL elements 12a-d. Each VCSEL element emits an uncollimated beam of laser light perpendicular to the VCSEL array surface. Because of limitations inherent in VCSEL technology, the emitting regions of the VCSEL elements cannot be arranged adjacent to each other, but must be arranged with some minimum spacing between nearest neighbors.
The light from the VCSEL array is collected by a focusing lens 14 and focused at a focal plane 16 where the light from the VCSEL elements 12a-d forms focused spots 18a-d, respectively. The pattern of illuminated spots at the focal plane is an image of the VCSEL array surface. Correspondingly, the focused spots are not adjacent to each other, but are arranged in a pattern that is geometrically similar to the pattern of laser elements on the VCSEL array.
From U.S. Pat. No. 4,428,647 it is known that this limitation on spacing of the focused spots from a laser array such as a VCSEL array can be solved by providing a complementary microlens array aligned to the laser array. The microlens array provides one lenslet focused on each VCSEL source, with lenslet diameters approximately equal to the VCSEL source spacing. In the apparatus of FIG. 2, a microlens array 20 is combined with the VCSEL array 10. The microlens array includes lenslets 22a-c arranged in a pattern with the same dimensions as the pattern of VCSEL elements 12a-c on the VCSEL array. However, the lenslets are larger in extent than the VCSEL elements, large enough to be adjacent on the surface of the microlens array.
The lenslet surface of the microlens array is spaced from the VCSEL array surface by a distance equal to the lenslet focal length. And each flenslet is position exactly above its corresponding VCSEL element. The divergent laser beam from VCSEL element 12a is collected and collimated by lenslet 22a. Similarly, the laser beams from VCSEL elements 12b and 12c are collected and collimated by lenslets 22b and 22c, respectively. The independent collimated laser beams 24a-c emitted through the lenslets are slightly divergent, because of optical diffraction through the lenslet apertures. But because the lenslets are substantially larger than the VCSEL elements, the divergence of the collimated laser beams is much smaller than the divergence immediately after the VCSEL elements.
The collimated laser beams are focused by a focusing lens 14 onto a focal plane 16 forming a set of focused spots 18a-c illuminated by the VCSEL elements 12a-c. The pattern of illuminated spots at the focal plane is an image of the microlens array. Therefore, the focused spots are essentially adjacent, with the same spatial pattern as the lenslets.
In order to create focused spots that are as small as possible, it is necessary to use a focus lens with a short focal length. The apparatus of FIG. 3 includes a smaller focus lens 14xe2x80x2 with short focal length to focus the laser beams 24a-c from a VCSEL array 10 collimated by a matched microlens array 20. Just as in FIG. 2, the VCSEL elements 12a-c each illuminate a separate focused spot 18a-c. But in FIG. 3, the spots are smaller and closer together, because the focus lens has a shorter focal length. However, because the focus lens 14xe2x80x2 has a smaller diameter than focus lens 14 in FIG. 2, the laser beams 24a and 24c from the outlying VCSEL sources 12a and 12c are not collected in their entirety by the focus lens. Hence, the illumination of the corresponding focused spots 18a and 18c is reduced by apodization. To collect all of the light from a large microlens array, the focusing lens diameter must be oversized by approximately the size of the VCSEL array compared to the lens size required to focus a single source. For applications that require very small focused spots, it may be impractical to provide such a large focusing lens.
A previously known solution to correct beam inefficiency due to apodization at the focus lens aperture is to provide a field lens or other external optics to steer the laser beams from all the VCSEL elements together at the objective lens aperture (see commonly assigned U.S. Pat. No. 5,745,153 and U.S. Pat. No. 5,808,986). FIG. 4 shows a positive field lens 26 combined with the VCSEL array 10 and matched microlens array 20. The field lens deflects the outlying laser beams 24a and 24c so that they converge with the central laser beam 24b at the aperture of a short focal length focus lens 14xe2x80x2. The laser beams form focused spots 18a-c. The positions and sizes of these spots are essentially unaffected by the field lens 26. And the outlying focused spots 18a and 18c have full intensity because laser beams 24a and 24c are not apodized at the focus lens. However, incorporation of a field lens results in a relatively complex optical structure.
It is an object of the present invention to produce an apparatus which makes use of a VCSEL array and which uses simplified optics.
This object is achieved by apparatus for collimating multiple laser beams from a Vertical Cavity Surface Emitting Laser (VCSEL) array and directing the collimated beams through a common aperture, comprising:
a) a VCSEL array including at least two VCSEL sources disposed in a spatial pattern and each VCSEL source emitting a divergent laser beam;
b) a microlens array with lenslet elements disposed in a spatial pattern geometrically similar to the pattern of sources on the VCSEL array, said lenslet pattern being scaled to a smaller dimensions than the VCSEL source pattern and arranged to receive the divergent laser beams; and
c) the microlens array being arranged so that the lenslet surface of the microlens array is maintained at a distance from the VCSEL array substantially equal to the focal length of the lenslets to substantially collimate the beams and also being maintained in a lateral orientation so that each beam passes through a corresponding microlens in the microlens array causing the laser beam from each VCSEL source to be directed through a common aperture.
In accordance with the invention, the microlens array itself may be adapted to steer the individual beams together at the objective lens aperture, eliminating the need of a field lens. The microlens elements are spaced slightly closer together than the VCSEL elements. Thus, the microlenses near the edge of the array work slightly off-axis and deflect the outer beams inward to common aperture. The requisite spacing change is approximately equal to the VCSEL source pitch times the ratio of the microlens focal length to the distance from the VCSEL assembly to the common aperture.
This invention is useful for any application in which an array of spaced, independent light sources is combined using a microlens array to form a close spaced array of focused light beams. By spacing the microlens elements closer together than the sources, the beams are substantially coincident at the aperture of a common focusing lens.